CONSTRAINING THE INITIAL MASS FUNCTION IN EXTREME ENVIRONMENTS: DETECTING YOUNG LOW-MASS STARS IN UNRESOLVED STARBURSTS

摘要

We demonstrate the feasibility of detecting directly low-mass stars in unresolved super star clusters with ages <8 Myr using near-infrared spectroscopy at modest resolution (R ～1000). Such measurements could constrain the ratio of high- to low-mass stars in these extreme star-forming events, providing a direct test of the universal nature of the initial mass function (IMF) compared to the disk of the Milky Way. We compute the integrated light of super star clusters with masses of 10~6 solar mass drawn from the Salpeter and Chabrier IMFs for clusters aged 1, 3, and 10 Myr. We combine, for the first time, results from Starburst99 for the main-sequence and post-main-sequence population (including nebular emission) with pre-main-sequence (PMS) evolutionary models for the low-mass stars as a function of age. We show that ～4%-12% of the integrated light observed at 2.2 μm comes from low-mass PMS stars with late-type stellar absorption features at ages ≤3 Myr. This light is discernible using high signal-to-noise ratio spectra (>100) at R = 1000, placing constraints on the ratio of high- to low-mass stars contributing to the integrated light of the cluster.